Mercury arc rectifier connection with parallel discharge paths



March 1, 1966 c. l. BOKSJO 3,238,417

MERCURY ARC RECTIFIER CONNECTION WITH PARALLEL DISCHARGE PATHS Filed Nov. 26. 1962 ,ad w awwzt United States Patent 3,238,417- IVIERCURY ARC RECTIFIER CONNECTION WITH PARALLEL DISCHARGE PATHS Carl Ingvar Boksjii, Lndvika, Sweden, assignor to Allmtinna Svenska Elektriska Aktiebolaget, Vasteras, Sweden, a Swedish corporation Filed Nov. 26, 1962, Ser. No. 240,004 Claims priority, application Sweden, Feb. 15, 1962,. 1,654/62 8 Claims. (Cl. 315-323) The present invention refers to a mercury arc rectifier connection with parallel discharge paths and provides a solution to the problem of producing a suitable distribution of the current between the separate discharge paths.

With current converters of a certain size, i.e. for certain high voltage and current strength, it is desirable to divide the current between a number of parallel discharge paths, whereby steps must be taken in order to obtain suitable current distribution between these. Hitherto it has been considered desirable to obtain working conditions as equal as possible in the parallel discharge paths, in other words a completely symmetrical distribution of the current and simultaneous ignition and extinction of all discharge paths.

The present invention ditfers from this principle in that it suggests different ignition times of parallel discharge paths, in such a way that when the current in the first discharge path has grown to a desired magnitude the next discharge path is ignited, etc. In order to make this possible, according to the invention impedances are inserted in the conductors to the diiferent discharge paths and a mercury arc rectifier connection according to the invention will therefore be characterised in that the ignition times for the different discharge paths are delayed according to a certain programme, which programme as well as the relation between the impedances inserted in the rectifier conductors, is adapted in relation to the desired current distribution between the difierent discharge paths.

In this connection it is suitable to distribute the impedances in the conductors in such a way that the impedance for the first-ignited discharge path is the largest, while the impedance decreases with increasing displacement of the ignition time. In this way the first-ignited discharge path will have the least current while the later igniting discharge paths have greater currents. The invention thus gives the advantage that only the firstignited discharge path is ignited at the full commutating voltage. The last-ignited discharge path does not need to be supplied with any impedance as no voltage is required for igniting any following discharge paths. As the last-ignited discharge path preferably has the largest current it is advantageous to avoid any impedances in series with this.

If the impedances in the rectifier conductors are inductive and the impedance distribution corresponds to the ignition sequence, the extinguishing sequence will be opposite to the ignition sequence because the current derivatives correspond to the inductances. In this way the deionising of the first-extinguishing discharge paths is improved.

A further advantage with the invention is that because the voltage concentrations at ignition of the later discharge paths in the sequence are very limited in relation to the first ignited discharge path, the voltage dividers for the later-ignited discharge paths could be dimensioned for these limited voltages.

From the above it is evident that the invention also allows equal or mainly equal current distribution between all discharge paths, in which case the series impedances, however, must be mainly inductive. Equal current distribution involves namely equal resistance distribution,

3,238,417 Patented Mar. 1', 1966 so that resistive series impedance in this case would cause undesirable resistive losses in all parallel current ways.

The invention is more closely described with reference to the attached drawing, where FIGURE 1 shows a mercury arc rectifier connection according to the invention, comprising two parallel discharge paths, while FIG- URE 2 shows a further development of the invention. For the sake of simplicity only two discharge paths are shown in both cases but as is clear from the above an arbitrary number of discharge paths could be inserted in the ignition sequence between the two shown.

FIGURE 1 shows a mercury arc rectifier connection comprising two parallel connected discharge paths 1 and 2. Both paths consist of an anode 11 and 21 respectively, a voltage divider 12 and 22 respectively with pertaining intermediate electrodes 13 and 23 respectively and a cathode 14 and 24. respectively. In addition each mercury arc rectifier includes a grid 15 and 25 respectively which maintains grid voltage from a grid voltage device 3. In the arrangement shown rectifier 1 is intended to be ignited first and is series connected with an impedance in the form of a reactor 4. The rectifier 2, which is intended to be ignited last, is not supplied with any series impedance.

At commutation the mercury arc rectifier 1 receives an ignition impulse and a current will grow up in it owing to the com-mutation voltage. When the current in the rectifier 1 has reached its desired level, the rectifier 2 receives an ignition impulse and has then an ignition voltage corresponding to the voltage over rectifier 1 in series with its impedance 4, 5.

The impedance 4 must be so dimensioned that the voltage over it is suflicient to ignite the rectifier 2 at the right moment. When the rectifier 2 has been ignited the current in rectifier 1 becomes constant, while the rest of the current is conducted by rectifier 2. At the next commutation, when the rectifiers 1 and 2 shall deliver their current to another rectifier connection, the rectifier 1 because of the inductance in the reactor 4 will retain its current practically unchanged until rectifier 2 is currentless. During the time when the current in rectifier 1 decreases to zero, rectifier 2 thus is currentless and only influenced by the voltage over the reactor 4, which gives a favourable deionisation of the rectifier 2.

In order to decrease the oscillations in the ignition current it is usual to supply mercury arc rectifiers with a damping arrangement comprising a so-called anode reactor parallel connected with a resistance. As these oscillations depend upon the breakdown of the commutation voltage at ignition and in the connection shown only rectifier 1 ignites at the full commutation voltage, it is sufficient to supply this rectifier with an anode clamping circuit 6, as indicated on the drawing. In this connection it may be favourable to incorporate the anode reactor in reactor 4 so that the conductor to rectifier 1 only contains one reactor. Further, with the arrangement shown it is easy .to arrange the anode reactor in the immediate vicinity of the anode 11, which is also an advantage.

With a connection as shown it may be desirable to let the last-ignited rectifier 2 take care of the greater part of the current, in which case the rectifier 1 only takes care of a little current and thereby preferably has the character of an auxiliary rectifier. Since, however, the rectifier 2 is not influenced by high voltages, during ignition or extinction, it is possible to load this rectifier relatively heavier than has been allowed for previously known connections, Where 'all the rectifiers must be capable of enduring full voltages during ignition and extinction. For the same reason an advantage is gained by constructing the voltage dividers 12, 13 and 22, 23 respectively with regard to their separate functions.

The idea of allowing one of the rectifiers to act as auxiliary rectifier may also be used with more than two rectifiers, in which case two or more of the later ignited rectifiers can be ignited simultaneously and then be supplied with impedances of the same size. The advantage is still that only the first rectifier ignites with the full commutation voltage.

In order to obtain suitable amplitude and durability of the voltage over the reactor 4, this can be parallel connected with a resistance, suitably a voltage-dependent resistance, a so-called varistor 5.

FIGURE 2 shows a further development of the arrangement according to FIGURE 1, where the impedance in the conductor to the first-ignited discharge path contains such a higher resistance that the discharge path is extinguished When the next parallel path ignites and thereby is connected in parallel with the first.

In this case also an anode damping circuit 6 is necessary. The discharge path 1 according to FIGURE 2 is thus a decided auxiliary rectifier, which only functions at ignition of the mercury arc rectifier connection. The impedance in the conductor to rectifier 1 consists here for example quite simply of a relatively high ohmic resistance 8 and the terms are otherwise the same as in FIGURE 1.

I claim:

1. Mercury arc rectifier connection comprising a pair of terminals, at least first and second parallel-connected unidirectional conducting discharge paths each connected between said terminals independently of the other discharge paths, and each provided with a grid control electrode; control means for said discharge paths; said grid control electrodes being connected to said control means; said control means including means for first delivering ignition pulses to the control electrode of said first discharge path and thereafter to the other control electrode; at least said first discharge path being connected in series with an impedance; said impedance being dimensioned in relation to the d sired current distribution between the different discharge paths; said control means including means to delay the ignition pulses to said other grid control electrodes according to said desired current distribution.

2. Mercury arc rectifier connection as claimed in claim 1; the first igniting discharge path having the greatest series impedance; said discharge path conducting least current of all said parallel connected discharge paths; said series impedances being still smaller in relation to the delaying of ignition of the corresponding discharge path.

3. Mercury arc rectifier connection as claimed in claim 2; the series impedance corresponding to the last ignited discharge path being substantially zero.

4. Mercury arc rectifier connection as claimed in claim 1; said series impedances being substantially inductive.

5. Mercury arc rectifier connection as claimed in claim 4; said inductive series impedances being parallel connected with resistors; said resistors being preferably voltage dependent resistors of solid material.

6. Mercury arc rectifier connection as claimed in claim 1; said series impedance corresponding to the first igniting of said discharge paths comprising a damping circuit; said damping circuit being arranged to damp out oscillations owing to the ignition of said discharge path.

7. Mercury arc rectifier connection as claimed in claim 1; said series impedance for the first ignited discharge path being so high that said discharge path is extinguished when the next discharge path is ignited.

8. Mercury arc rectifier connection as claimed in claim 1; each of said discharge paths being provided with its own intermediate electrodes connected to a voltage divider for said discharge path; said voltage divider being dimensioned according to the operation conditions of said discharge path.

References Cited by the Examiner UNITED STATES PATENTS 2,254,703 9/1941 Knight 315-181 2,481,468 9/ 1949 Bridges 315258 2,497,166 2/1950 Goldberg et al. 315252 X 2,527,211 10/1950 Brinson 315214 2,549,807 4/1951 Heed 32l37 X 2,937,318 5/1960 Chao 315-846 3,014,157 12/1961 Dillon et al 32138 X FOREIGN PATENTS 1,296,032 5/ 1962 France.

GEORGE N. WESTBY, Primary Examiner.

ARTHUR GAUSS, Examiner.

R. JUDD, K. CROSSON, Assistant Examiners. 

1. MERCURY ARC RECTIFIER CONNECTION COMPRISING A PAIR OF TERMINALS, AT LEAST FIRST AND SECOND PARALLEL-CONNECTED UNIDIRECTIONAL CONDUCTING DISCHARGE PATHS EACH CONNECTED BETWEEN SAID TERMINALS INDEPENDENTLY OF THE OTHER DISCHARGE PATHS, AND EACH PROVIDED WITH A GRID CONTROL ELECTRODE; CONTROL MEANS FOR SAID DISCHARGE PATHS; SAID GRID CONTROL ELECTRODES BEING CONNECTED TO SAID CONTROL MEANS; SAID CONTROL MEANS INCLUDING MEANS FOR FIRST DELIVERING IGNITION PULSES TO THE CONTROL ELECTRODE OF SAID FIRST DISCHARGE PATH AND THEREAFTER TO THE OTHER CONNECTED; AT LEAST SAID FIRST DISCHARGE PATH BEING CONNECTED IN SERIES WITH AN IMPEDANCE; SAID IMPEDANCE BEING DIMENSIONED IN RELATION TO THE DESIRED CURRENT DISTRIBUTION BETWEEN THE DIFFERENT DISCHARGE PATHS; SAID CONTROL MEANS INCLUDING MEANS TO DELAY THE IGNITION PULSES TO SAID GRID CONTROL ELECTRODES ACCORDING TO SAID DESIRED CURRENT DISTRIBUTION. 